1. Field of the Invention
This present invention relates to a composition for developing photosensitive coatings. More specifically, the present invention relates to a method and composition for developing lithographic printing plates and similar photographic elements. More particularly, the invention relates to aqueous developers suitable for removing the non-image area of negative-working photographic elements.
2. Description of Related Art
The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
The most common type of lithographic plate to which the present invention is directed has a light sensitive coating applied to an aluminum base support. The coating may respond to light by having the portion which is exposed become soluble so that it is removed in the developing process. Such a plate is referred to as positive acting. Conversely, when that portion of the coating which is exposed becomes hardened the plate is referred to as negative acting. In both instances the image area remaining is ink receptive or oleophilic and the non-image area or background is water receptive or hydrophilic. The differentiation between image and non-image areas is made in the exposure process where a film is applied to the plate with a vacuum to insure good contact. The plate is then exposed to a light source having a portion of which is composed of UV radiation. In the instance where a positive plate is used, the area on the film that corresponds to the image on the plate is opaque so that no light will strike the plate whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating which then becomes more soluble and is removed. In the case of a negative plate the converse is true. The area on the film corresponding to the image area is clear while the non-image area is opaque. The coating under the clear area of film is hardened by the action of light while the area not struck by light is removed. The light hardened surface of a negative plate is therefore oleophilic and will accept ink while the non-image which has had the coating removed through the action of a developer is desensitized and is therefore hydrophilic.
The present invention provides a new aqueous developer for negative working lithographic printing plates. Most developer compositions for negative working printing plates disadvantageously contain organic solvents which are offensive through their volatility and high concentration, pose toxic exposure hazards to the end user and are pollutants to the environment when evaporated into the air and/or discharged into public waterways. Presently there is a trend going toward aqueous developers. These aqueous developers also contain organic solvents at high concentrations which are still environmentally harmful and toxic to the end user. The prime advantage is that the solvents are not odor-wise offensive. They have the disadvantage of having a strongly alkaline pH, cause redeposit on the plate and machine parts, and solubilize the coating residue which creates disposal problems and reduces the effective life of the developer in a processing machine. The present invention provides a composition which substantially alleviates these problems.